1. Field of the Invention
This invention relates to a polyisocyanate preparation dispersible in water based on an aliphatic polyisocyanate or a mixture of aliphatic polyisocyanates, as well as the use of this polyisocyanate preparation as an additive for aqueous adhesives based on polymers dispersible in water.
2. Description of the Prior Art
It is known to use polyisocyanates as additives for adhesives based on polymers dissolved in organic solvents. Thus, for example, polyfunctional polyisocyanates are added to solutions of natural or synthetic rubber or solutions of polyurethanes to achieve a more favorable adhesion on many materials to be bonded, a higher heat resistance of the bond and a better chemical resistance. According to the teachings of EP-A-0,061,628, the adhesive properties of aqueous polymers dispersible in water are improved by the addition of hydrophilically modified aromatic polyisocyanates.
This improvement is not sufficient, however, to achieve load B4 according to DIN 68,602 as required by the market. Wood bonds which are produced with polyvinylacetate adhesives according to the teaching of EP-A-0,061,628 can thus not be used indoors where there are extreme climatic fluctuations and the influence of water (indoor swimming pools, shower cubicles), nor can they be used outdoors due to vast climatic influences (for example windows, exterior doors, ladders, steps). These uses have only been possible by the addition of toxic producing chromium (III) salts; however, these salts lead to a sharp and undesirable drop in the viscosity of the adhesives. A further disadvantage of the adhesives modified according to the teaching of EP-A-0,061,628, for example those based on aqueous polyurethane dispersions, is the heat resistance of the resulting bonds at least about 75.degree. C. (c.f. the comparative example below) which is too low for many practical applications. If such modified polyurethane dispersion adhesives are used, for example, with PVC film for laminating the wood fiber board in the production of automotive side parts, then this bond must withstand heat storage at at least 90.degree. C. without stress.
It is thus the object of the present invention to provide new polyisocyanate preparations for use in aqueous adhesives which better fulfill the required adhesive properties and heat resistance of bonds than the present preparations.
This object can be achieved with the polyisocyanate preparations according to the invention based on aliphatic polyisocyanates and described in more detail below. With the polyisocyanate preparations according to the invention, it is possible, for example, to improve the conventional aqueous adhesives based on polyvinylacetate such that the resulting bonds correspond to load group B4 according to DIN 68 602 even without the simultaneous use of chromium (III) salts. Further, with the polyisocyanate preparations according to the invention, it is possible to improve aqueous adhesives based on polyurethane dispersions so that bonds produced therewith have the heat resistance required in practice at temperatures of about 90.degree. C. This is surprising since previously only polyfunctional aromatic polyisocyanates have been added to the conventional solvent-containing adhesives based on natural and synthetic rubber or based on polyurethanes to improve the adhesive properties of the resulting bonds. A further advantage of the polyisocyanate preparations according to the invention is that the aqueous adhesives modified with the inventive preparations have a substantially longer pot life than corresponding adhesives containing the polyisocyanate preparations of the prior art based on aromatic polyisocyanates.